The most common and most likely answer to this crossword clue is the 7 letter word LUDMILA
I've seen this crossword clue in Financial Times
L
U
D
M
I
L
A
Buddy explains!
The straight part
We know that "Czech saint" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case czech saint = ludmila.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.